Undesirable deformations of stiffness--critical structural components under load can be caused by tension, compression, bending, and torsion (twisting). The largest deformations are due to bending since it is causing angular distortions in the component being deformed and thus displacements of the remote points of the component can be amplified by projections of these angular distortions. Especially pronounced these displacements are in cantilever components which have long unsupported spans.
It is known from the Strength of Materials that stiffness of a structural component (i.e., the magnitude of force, moment, or torque needed to generate a unit of deformation) is determined by its geometry (for bending--cross sectional moment of inertia, position of application point of the forcing factor, and position of the point in which the deformation is measured) and by modulus of elasticity (Young's modulus) of the material. While the geometry is usually determined by the design considerations, Young's modulus is constant for a given family of materials and cannot be changed by alloying or by heat treatment, contrary to strength properties. For example, while yield strength values for high strength steel, high strength aluminum brands are up to about ten times higher than the yield strength values for ordinary steel, aluminum brands, respectively, values of Young's moduli for both high strength and ordinary brands are essentially the same. As a result, enhancement of stiffness requires either "beefing up" of structural designs thus increasing their dimensions, weight, and cost, or use of expensive materials having high Young's moduli (e.g., sintered tungsten carbide), which usually also have higher density and lead to increasing weight of the structural components. If stiffness enhancement is needed for increasing values of natural frequencies of the structure, weight increase can negate effects of the enhanced stiffness. In many cases design parameters of the structure are sacrificed since the desired stiffness cannot be achieved within the prescribed parameters of size and weight. In some instances, low stiffness can be compensated by active (servo-controlled) systems, which add cost, complexity, and weight to the structure while having their own performance limitations and reliability problems, and also require a constant energy supply for operation.
It is known that bending stiffness of slender components can be enhanced without changing their geometry and material by judicious application of forces. An example of such approach is a string of string musical instruments, e.g. guitar. Stretching of the string leads to a higher pitch (higher natural frequencies) without changing its mass, thus it effectively increases bending stiffness of the string. While prestressing (the "guitar string" effect) is used in design practice to enhance structural stiffness, its application is limited since in many cases external forces cannot be continuously applied to structural components, especially to cantilever components. Some examples of self-contained systems in which permanent tension of external parts of a structural component is compensated by permanent compression of its internal parts are given in the book by E. Rivin, "Mechanical Design of Robots", McGraw-Hill, 1988. In one example the internal part is a rod which can be compressed by using a threaded connection, thus causing stretching of the external part. In this system bending stiffness of the external part is increasing while bending stiffness of the internal part (rod) is decreasing due to their stretching, compression, respectively. However, since bending stiffness of the internal layers of a beam does not contribute significantly to its overall stiffness, the overall stiffness of the beam is increasing.
Some of shortcomings of such self-contained systems are their complexity and also a danger of buckling of the internal parts under compressive forces.
Another shortcoming is a difficulty to apply a desired degree of preload since it requires a precise deformation of the internal member.
Yet another shortcoming is a difficulty to use this technique to a beam with a complex (not round or square) cross section.
The present invention addresses the inadequacies of the prior art by providing a method for enhancement of beam stiffness in bending by using transformations (thermal expansion, phase transformation, chemical changes) of a medium filling the internal space of the beam. Since these transformations are usually accompanied by volume changes, volume increase of the media locked in the enclosed space inside the beam results in generating of tensile stresses in the surrounding beam structure and thus, in the stiffness increase.
These and other advantages of the present invention will be readily apparent from the drawings, discussion, and claims which follow.